I was born in 1964, and as a child of the Seventies, the national flag of
my country (the UK) was universally known as the Union Jack. For example:
"I won't wear the Union Jack", by Tracey Curtis (also a child of the
Seventies).

But then, some years later, some pedant wrote in to the BBC about a report
in which mentioned the Union Jack, and claimed that the flag should only
be referred to as the Union Jack when flown on a ship.

Since then, all media has referred to the flag as the Union Flag, a name
which no one uses in real life.

From my brief investigation on the web (i.e. Wikipedia) it seems that the
names Union Flag and Union Jack are interchangeable, and both are
historically correct. However, the belief that the flag should only be
referred to as the Union Jack when flown at sea is *a rumour*! In fact,
the name Union Jack pre-dates the flying of the flag on the jack staff.

I'm on the side of the writer, I must admit - the "Flag not Jack" thing is the hallmark of a pedant, and when one looks in to these things, one often finds that the pedant is wrong. In general, a jack is a naval flag, but I don't think that pedantry in the face of common usage is particularly productive.

I have no evidence beyond that, but there was an article about our revised citizenship test for immigrants on the BBC a few weeks ago, with samples of typical new multiple choice questions:

In 1801, a new version of the official flag of the United Kingdom was created. What is it often called?(The British Standard, the Royal Banner, the St George Cross, the Union Jack)

If you read the comments, there are loads of pedants, claiming (wrongly as usual) that there's no correct answer to this question because it's "the Union Flag". It was a surprisingly long time before a more accurate, less pedantic person (not me) pointed out that the question says "What is it often called?", so "the Union Jack" is a perfectly good answer. And that the ability to read is probably more useful than pedantry to citizens (sorry, I mean "subjects") in general.

That question is worded to avoid the issue, though, because there isn't an official answer as far as I know.

It seems the default is to call it the Union Flag, but a debate in Parliment regarding where it was okay to fly it called it a Union Jack even when flown on land. The Flag Institute (which is linked to from the UK governments culture site) says it has been called the Union Jack on land and sea for a long time.

It is often stated that the Union Flag should only be described as the Union Jack when flown in the bows of a warship, but this is a relatively recent idea. From early in its life the Admiralty itself frequently referred to the flag as the Union Jack, whatever its use, and in 1902 an Admiralty Circular announced that Their Lordships had decided that either name could be used officially. Such use was given Parliamentary approval in 1908 when it was stated that "the Union Jack should be regarded as the National flag".

If there are any official laws regarding our flag, ideally they would refer to it as "the flag of the United Kingdom of Great Britain and Northern Ireland" to allow for the design to change without having to rewrite all the laws. ... You'd probably need a place holder for the name of the country, as well, come to that.

It is worth pointing out that the Admiralty statement that Richard mentioned in his last post covered not only when flying the Union Flag/Jack from ships, but for any use. [See a fairly recent QI programme.]

This issue has been covered by letters and short articles in BBC History Magazine over the years. One article pointed out that the term Union Jack was first used in the 1640s (or about then) before either Great Britain or the United Kingdom had been created. James I of England/VI of Scotland had the flag created to show that he was the first king of England and Scotland - or the King of Great Britain as he termed himself. Anything to do with James I/VI's reign is referred to as Jacobean (furniture, architecture, theatre, etc) and it is thought that the word Jack could be a corruption of Jacobean.

The Australian Flag Act (1953) sets in law the design for the Australian flag. It specifically says that "the Union Jack occupies the upper quarter next to the staff". So the British flag is enshrined in Australian law as the Union Jack.

The jack shall consist of a blue field charged with a compound saltire (crossing) of alternate tincture white and red, the white having precedence; a narrow edge of white borders each red side of the saltire;

I said earlier that the term Union Jack may refer to a corruption of the word Jacobean referring to James I who had the flag created. I have found support for this on the official website of the UK Parliament:

Quote:

Union Jack flag

In 1606 he gave orders for a British flag to be created which bore the combined crosses of St George and of St Andrew. The result was the Union Jack, Jack being a shortening of Jacobus, the Latin version of James.

Actually, the Union Jack/Flag was not flown from ships; instead they fly ensigns; with the Union flag design forming only a quarter of the flag (like the stars in the Stars and Stripes). The colour of the rest of the flag depends on who owns the ship; red cross with white in the other three quarters for Naval Vessels, Blue for other goverment/ naval reserve ships and Red for civilian vessels.
These are normally known as White, Red or Blue Ensigns (or Dusters amongst sailors), and there are also (rarely used) Jack versions of the Ensigns which are the same design but in square format.
The Union Flag may be used in civilian vessels, or on Naval vessels in wartime (known as the Battle Ensign).

But what do vessels of the RN fly from the jackstaff (on the bow) when moored or at anchor? On commissioned USN ships we fly the national ensign from the mast underway and from the flag staff at the stern when in port, but we additionally fly the "traditional" (read: not really) first navy jack from the jack staff. Prior to 2002 the navy jack was just the 50 stars on a blue field as seen in the upper left corner of the US flag.

But what do vessels of the RN fly from the jackstaff (on the bow) when moored or at anchor? On commissioned USN ships we fly the national ensign from the mast underway and from the flag staff at the stern when in port, but we additionally fly the "traditional" (read: not really) first navy jack from the jack staff.

Royal Navy vessels fly the White Ensign from the mast while underway:

And they fly the Union Jack from the jackstaff while in port:

ETA: I saw that Mycroft said Royal Navy ships don't fly the Union Jack so I checked online and the Ministry of Defence have a document about flag rules on their website that says, "The Union Flag is to be worn as a Jack... during the hours laid down in Para 9136, by ships in harbour, at anchor, secured to a buoy or alongside, but not when in dock."